Traditional laminated panels, intended to be used for e.g. flooring or furniture components, are generally produced by the following steps:                applying a melamine formaldehyde resin impregnated paper as balancing layer on one face of a core of wood fibre based material, for example, HDF;        applying a melamine formaldehyde resin impregnated printed decor paper on the other face of the core;        applying a melamine formaldehyde resin impregnated transparent overlay paper with wear resistant particles, for example, aluminium oxide, on the décor paper; and        curing the resin by applying heat and pressure in a continuous or discontinuous press to obtain a laminated product.        
Typical press parameters are 40-60 bar pressure and a temperature of 160-200° C. with a pressing time of 8-45 seconds. The surface layer has generally a thickness of 0.1-0.2 mm, the thickness of the core varies between 6-12 mm and the balancing layer is about 0.1-0.2 mm thick.
This production method and product produced by such methods are generally referred to as the DPL process and DPL products (Direct Pressure Laminate).
Recently, building panels with a wood fibre based surface have been developed. Powder comprising wood fibres, a binder, preferably melamine formaldehyde resin, aluminium oxide particles and colour pigments are scattered on a core of a wood fibre based material such as HDF and pressed under heat and pressure in a continuous or discontinuous press to obtain a product with a paper free and solid surface layer. The press parameters are similar to DPL. A higher pressure of 40-80 bar and a pressing time of 15-45 seconds may be used when the surface is formed with deep embossing and a thickness of, for example, 0.4-0.6 mm. The pressing temperature is generally 150-200° C. Such wood fibre based floors, generally referred to as Wood Fibre Floors (WFF), have considerably better properties than traditional laminate floors since a thicker and more impact-resistant and wear-resistant surface with deep embossing may be produced in a cost efficient way.
Both DPL products and panels having a wood fibre based surface have a balancing layer arranged on the rear face of the core. The balancing layer may be a balancing paper or powder-based balancing layer comprising wood fibres and a binder. The core with the upper and lower layers is moved into a press and pressed under heat and pressure such that the layers are cured and attached to the core.
The layers on the front face and the rear face of the core are exposed to a first shrinking when the thermosetting resin in the upper and lower layer cures during pressing. The balancing layer at the rear face balances the tension that is created by the surface layer of the front face and the panel is substantially flat with a small convex backward bending when it leaves the press. Such first shrinking and balancing of the panel is below referred to as “pressing balancing”. The second temperature shrinking, when the panels is cooled from about 150-200° C. to room temperature, is also balanced by the balancing layer and the panel is essentially flat. The second balancing is below referred to as “cooling balancing”. A small convex backward bending is preferred since this counteracts upward bending of the edges in dry conditions when the relative humidity may go down to 20% or lower during wintertime.
A problem is that this essentially flat panel comprises tension forces caused by the shrinking of the surface and balancing layers during pressing and during cooling to room temperature.
The surface layer and the core will swell in summertime when the indoor humidity is high and shrink in wintertime when the indoor humidity is low. The panels will shrink and expand and a cupping of the edges may take place. The balancing layer is used to counteract such cupping. In the installed floor, the balancing layer is used to work as a diffusion barrier for moisture from the underlying floor, and to minimize the impact of the surrounding climate. Consequently, the balancing layer is adapted balance shrinking and expansion caused by both pressing, cooling and climate changes.
It is known that a balancing layer comprising a powder mix of wood fibres and a thermosetting binder can be used to balance the surface layer. A method for producing such a balancing layer is described in WO 2012/141647.
US 2010/0239820 describes a method of producing a laminated decorative plate wherein an unimpregnated decorative layer is applied onto a layer of glue arranged on the core. A counteracting layer is arranged on the other side of the core, and it is described that neutral paper with solid or liquid resin as a glue is suitable for the counteracting layer. The counteracting layer is adapted to prevent the finished plate from warping or bending as a result of influences of temperature and humidity.
WO 2010/084466 discloses a balancing layer substantially consisting of a synthetic material layer, which is free from material sheets. The synthetic material is provided in liquid form.